1. Field of Invention
The invention relates to an image forming apparatus, in which an image is first formed on an intermediate medium using phase-changeable ink, such as hot melt ink, and then transferred onto a recording medium. More particularly, it is related to an image forming apparatus that is capable of forming the image onto both sides of the recording medium by accurately controlling the surface temperature of the recording medium.
2. Description of Related Art
For the image forming apparatus whose printing method is ink jet printing, it is necessary to maintain a gap between ink jet heads and the side of the recording medium to be recorded with the image in order to obtain high-quality images. However, especially for the image forming apparatus whose printing method is ink jet printing, in which a final ink image is directly formed onto the recording medium, such as paper, it is difficult to maintain the gap between the heads and the recording medium constant because the gap is significantly changed if the thickness of the recording medium is changed. For the image forming apparatus whose printing method is thermal printing, changes in the size of the recording medium, in particular, for color printing using multicolor thermal heads, causes a problem that the ink images formed by the each of the heads is out of position.
Therefore, there have been proposed various image forming apparatuses whose printing methods are an intermediate transfer printing in which an intermediate image is temporarily formed on the intermediate medium by selectively heating a phase-changeable ink. Then the intermediate image is transferred onto a recording medium, such as paper, by heating. An image forming apparatus using the intermediate transfer method can maintain the gap between the ink jet heads and a recording medium easily and obtain high-quality colored images. Moreover, an image can be positioned easily because the size of the paper does not change, i.e., does not shrink or wrinkle, due to heating.
However, the intermediate transfer method also causes a problem. For a method in which an ink image formed on the intermediate medium is heated and then transferred onto the recording medium, the ink image to be transferred onto the recording medium loses its shape because the ink image expands on the intermediate medium when it is heated.
For example, Japanese Patent Application Publication No. 7-276621 discloses an image forming apparatus with the intermediate transfer method in which the recording medium is heated rather than an ink image. First, the recording medium is heated and then the recording medium is contacted with the intermediate medium under pressure so that the ink image is transferred onto the recording medium. Because the ink image is not heated directly in this method, the ink image does not lose its shape on the intermediate medium.
However, the image forming apparatus described above also has some problems. In order to transfer the ink image onto the recording medium from the intermediate medium, a surface temperature of the intermediate medium side of a recording medium needs to be heated to a temperature equal to or higher than the softening point of the phase-changeable ink and also to a temperature equal to or higher than a specified temperature range (appropriate temperature range for transferring the phase-changeable ink). However, because the recording medium is heated from the opposite side from the side to be recorded with the intermediate image, the recording medium must be heated at higher temperature than if the recording medium was heated from the intermediate side. In this case, the side to which the ink image is to be transferred is heated through the recording medium, that is, the side that does not receive the ink image is also heated, so that the heat is wasted.
Because the intermediate medium side of the recording medium is heated from the opposite side from the intermediate medium side, it takes time to transfer heat to the intermediate medium side so that throughput is degraded.
Moreover, as a large amount of heat is applied to the recording medium, shrinking or deforming on the recording medium is likely to occur.
Furthermore, when double-sided printing is performed, another problem occurs. One side of the recording medium already has an image so that a new image is formed on the other side of the recording medium. As described above, the recording medium is heated from the opposite side from the side to be recorded the image so that the side which already has the image is heated again, that is, the phase-changeable ink forming the image is melted again. Because of this, dots forming the image are disordered or the image is transferred onto a transfer roller so that the image is damaged and the image forming apparatus and/or following recording medium get dirty, even if a high-quality image is formed on the opposite side from the side to be recorded the image. As a result of this, double-sided printing can not be performed on such image forming apparatuses.
It is an object of the invention to provide an image forming apparatus with which efficient printing can be performed such that the intermediate image using phase-changeable ink formed on the intermediate medium is transferred onto the recording sheet with less heat, double-sided printing can be performed, and high-quality images can be obtained without any damage from heat.
In order to achieve the above and other objectives, an image forming apparatus comprises an intermediate medium holding an ink image on a surface thereof, an image forming unit forming an intermediate image onto the intermediate medium using phase-changeable ink, a heating unit heating a recording medium, a press unit that brings the recording sheet being heated into contact with the intermediate image being held on the intermediate medium, and a transfer unit that transfers the intermediate image formed on the intermediate medium onto the recording sheet as a final image, wherein the heating unit heats a side to be transferred the final image of the recording medium.
The image forming apparatus which includes the structures as described above does not harm the intermediate image formed on the intermediate medium and, besides, the heating unit can heat the recording medium efficiently so that the image can be transferred with less heat because the heating unit is provided right above the recording medium in order that the side to be transferred the final image of the recording medium is heated directly. Therefore, the consumption of energy is reduced. Moreover, as described above, the recording medium is heated directly, the recording medium is heated for only a short time, therefore, throughput of the image forming apparatus can be increased and the shrink or size change on the recording medium, such as a recording sheet, is restrained. Furthermore, even though the recording medium already has images on one side using a phase-changeable ink, a new image can be formed on the other side of the recording medium without softening or melting the first images.
In one aspect of the image forming apparatus of the invention, the heating unit heats the recording medium. Heat quantity of the heating unit meets a formula xcex(Tsxe2x88x92Tr)/t equal to or more than Q equal to or more than {Cpt(Tsxe2x88x92Tr)/S}+{xcex(Tmxe2x88x92Ts)/t} when heat quantity per unit area which the heating unit applies to the recording medium is Q (W/m2), heating time which the heating unit heats the recording medium is S (s), heat conductivity of the recording medium is xcex(W/mxc2x7K), specific heat of the recording medium is C (J/kgxc2x7K), density of the recording medium is xcfx81 (g/m3), the thickness of the recording medium is t (m), melting point of the phase-changeable ink is Tm (degrees Celsius), softening point of the hot melt ink is Ts (degrees Celsius), and the room temperature is Tr (degrees Celsius).
The heat quantity of the heating unit can be controlled in order to prevent an image from being damaged such that the side of the recording medium to be transferred the image is heated to the temperature equal to or higher than the melting point of the phase-changeable ink so that the image can be transferred onto the recording medium and the opposite side to the side to which the image of the recording sheet to be transferred is prevented from overheating so that the solidified image is not softened. Therefore, a high-quality image can be printed on both sides of the recording medium.
In another aspect of the image forming apparatus of the invention, the heating unit comprises a temperature sensor that is capable of taking a surface temperature of a side heated by the heating unit of the recording medium.
The heat unit can heat the recording medium at the proper temperature based on a temperature which is sensed by the temperature sensor because the surface temperature of the heated side of the recording medium can be taken. Therefore, high-quality image can be formed onto the recording medium without wasting heat.
In other aspect of the image forming apparatus of the invention, the heating unit includes a temperature sensor that is capable of taking a surface temperature of a side which is opposite to the side heated by the heating unit of the recording medium.
The heat unit can heat the recording medium at the proper temperature based on a temperature which is sensed by the temperature sensor because the surface temperature of a side which is opposite to the side heated by the heating unit of the recording medium can be taken. Therefore, because the surface temperature of the side which is not heated directly by the heating unit can be taken, a new image can be transferred onto the recording medium without damaging the solidified image on the other side. In particular, the temperature sensors are provided where they can take the temperature of both sides of the recording medium so that a high-quality image can be printed on both sides of the recording medium.